Depending on the ultimate system you'll be creating, you may have a ton of work to do or you may have very little. In any case, we recommend that you start with a supported evaluation board. This approach minimizes the amount of low-level work that you have to do initially, thereby allowing you to focus on your system rather than on implementation details.
Start with an evaluation platform that most closely resembles your target platform — there are many supported evaluation platforms from various vendors.
Once you're comfortable with the development environment and have done a very rudimentary "proof of concept," you can move on to such development efforts as creating your own hardware, writing your own IPL and startup code, writing drivers for your hardware, and so on.
Your proof of concept should address such issues as:
Once these are addressed, you can then decide on your plan of attack.